Investigation of trace amounts of gas on microvave water-cut measurement

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In recent years, the upstream oil and gas industry has dealt with some of the most challenging metering applications. One of these is the measurement of water percentage at the point of allocation. It is an essential requirement when test separators or the newly developed full multiphase meters are utilized for oil well production testing. Water-cut can be obtained from measurement of differential pressure, capacitance/conductance, gamma rays absorption, absorption of infrared light, coriolis mass measurement, or microwave permittivity. The use of microwave permittivity has been shown to be very effective with the added benefit of not requiring a nuclear source, as is the case with a gamma ray densitometers. A common problem encountered in oil well production testing is that of gas Âcarry-underÂ into the liquid stream exiting the test separator. This results in a trace amount of gas entering the water-cut meter, producing errors in the water-cut reading. Gas carry-under may be caused by high liquid viscosity, improper separator operation, or poor separator design. Gas carry-under is believed to be one of the major causes of large allocation factors in oil and gas operations. Problems in clearly defining the three-phase stream as to flow regime and actual gas bubble size have been described in the technical literature. Pertinent references are discussed and compared. The issues in trying to perform such tests in the laboratory and the correlation of the data are disclosed and the difficulties in trying to correlate the effects of the entrained gas are described. Field testing and experience by at least one manufacturer of equipment has verified the effect of entrained gas, but little quantitative data relating gas-cut to increased error of measurement has been published. The objective of this work was to investigate the performance of a microwave water-cut analyzer under three-phase flow conditions to determine the impact of the presence of gas in the liquid stream. Experiments were performed that investigated the effects of entrained gas on a commercial water-cut analyzer. These tests were conducted at the Texas A&M Tommie E. Lohman Fluid Measurement Laboratory at low pressure conditions (< 40 psig). The test fluids were air, water and two types of oil: mineral oil and hydraulic oil. These experiments investigated oil continuous emulsion conditions with the Gas Volume Fraction (GVF) ranging from 0-25% and the water-cut ranging from 5-30%. Liquid flow rates were between 500-3,700 bbl/day. A 2-inch water-cut full range meter was utilized for these tests. The error in water-cut was seen to increase with increasing GVF ranging from 0% to 25%. However, the measurement remained stable over the entire range of tests. A correction was developed to correct water-cut meter readings based on the amount of gas in the liquid stream.